1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with a perpendicular magnetic recording layer for use in magnetic recording hard disk drives.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. The most common type of perpendicular magnetic recording system is one that uses a recording head with a single write pole and a “dual-layer” media, as shown in FIG. 1. The dual-layer media includes a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. This type of system is also called “Type 1” perpendicular magnetic recording. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording disk. The disk includes a hard disk substrate, an adhesion or onset layer (OL) for growth of the SUL, the SUL, an underlayer (UL) on the SUL to facilitate growth of the RL, and a protective overcoat (OC). One type of conventional material for the RL is a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy. The ferromagnetic grains of this material have a hexagonal-close-packed (hcp) crystalline structure and out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of the hcp crystalline structure being induced to grow perpendicular to the plane of the layer during deposition. To induce this epitaxial growth of the hcp RL, the UL onto which the RL is formed is also typically an hcp material. Ruthenium (Ru) is one type of material proposed for the UL. While a single-layer UL is depicted in FIG. 2, the UL may be a multilayer structure, with one or more hcp layers and a seed layer between the SUL and the hcp layer or layers. The UL also typically functions as an exchange break layer to break the magnetic exchange coupling between the RL and the magnetically permeable SUL.
A perpendicular magnetic recording medium has also been proposed wherein the RL is the upper ferromagnetic layer of an antiferromagnetically-coupled (AFC) layer, as depicted in FIG. 3. The AFC layer comprises a lower ferromagnetic layer on the UL, an antiferromagnetically (AF) coupling layer on the lower ferromagnetic layer, and an upper ferromagnetic layer (the RL) on the AF-coupling layer. In this type of medium, as described in U.S. Pat. No. 6,815,082 B2, each ferromagnetic layer is a granular cobalt alloy with perpendicular magnetic anisotropy. The AF-coupling layer induces perpendicular antiferromagnetic exchange coupling between the two ferromagnetic layers, as depicted in FIG. 3 by the antiparallel magnetization directions between the two ferromagnetic layers in each magnetized region of the AFC layer.
To achieve high performance perpendicular magnetic recording disks at ultra-high recording densities, e.g., greater than 200 Gbits/in2, the RL should exhibit low intrinsic media noise (high signal-to-noise ratio or SNR), a coercivity Hc greater than about 5000 Oe and a nucleation field Hn greater (more negative) than about −1500 Oe. The nucleation field Hn is the reversing field, preferably in the second quadrant of the M-H hysteresis loop, at which the magnetization begins to drop from its saturation value (Ms). The more negative the nucleation field, the more stable the remanent magnetic state will be because a larger reversing field is required to alter the magnetization.
It is well-known that the granular cobalt alloy RL should have a well-isolated fine-grain structure to produce a high-Hc media and to reduce inter-granular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the RL has been proposed by the addition of metal oxides which precipitate to the grain boundaries. The addition of SiO2 to a CoPtCr granular alloy by sputter deposition from a CoPtCr—SiO2 composite target is described by H. Uwazumi, et al., “CoPtCr—SiO2 Granular Media for High-Density Perpendicular Recording”, IEEE Transactions on Magnetics, Vol. 39, No. 4, July 2003, pp. 1914-1918. The RL described in this reference had Hc of about 4000 Oe and Hn of about −700 Oe. The addition of Ta2O5 to a CoPt granular alloy is described by T. Chiba et al., “Structure and magnetic properties of Co—Pt—Ta2O5 film for perpendicular magnetic recording media”, Journal of Magnetism and Magnetic Materials, Vol. 287, February 2005, pp. 167-171. The RL described in this reference had Hc of about 3000 Oe when the RL was sputter deposited from a composite target of CoPt and Ta2O5, and no increase in Hc was obtained by introducing oxygen gas during sputtering.
In the above-cited references the amount of oxygen added is not significantly greater than that required for the stoichiometric metal oxide. The effect of the addition of an even greater amount of oxygen to a CoPtCr granular alloy by reactive sputter deposition of a CoPtCr—SiO2 composite target in an argon/oxygen (Ar/O2) gas mixture is described by M. Zheng et al., “Role of Oxygen Incorporation in Co—Cr—Pt—Si—O Perpendicular Magnetic Recording Media”, IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, pp. 2498-2500. This reference teaches that the maximum Hc of about 4000 Oe is achieved at the optimal amount of oxygen in the RL of 15 atomic percent (at. %). Only a minor portion of the Cr is in the oxide form and there is no strong evidence of SiO2 in the RL. If the amount of oxygen is increased above the optimum, the excess oxygen forms oxides of Cr and Co in the grains, resulting in a reduction in Hc. At a level of 21 at. % oxygen, Hc is reduced to about 1000 Oe, which renders the RL unusable.
What is needed is a perpendicular magnetic recording disk with a CoPtCr granular alloy RL that exhibits Hn greater than about −1500 Oe and that has a well-isolated fine-grain structure resulting in high SNR and Hc greater than about 5000 Oe.